I feel terrible, today when I prayed ( in the final tasshahud) I accidentally insulted the prophet , I ask forgiveness from Allah and then I’ve retaken them prayer , does this count as a sin that Allah won’t forgive?
I'm not a scholar, so take what I say with a grain of salt.
In general, according to Islam, actions are judged according to the intentions.
On the authority of Ameer ul-Mu’mineen (the Commander of the Faithful), Abu Hafs `Umar ibn al-Khattab radiAllahu anhu, who said: I heard the Messenger of Allah sallAllahu alayhi wa sallam say:
“Actions are but by intentions and every man shall have only that which he intended. Thus he whose migration (Hijrah to Madeenah from Makkah) was for Allah and His Messenger, his migration was for Allah and His Messenger, and he whose migration was to achieve some worldly benefit or to take some woman in marriage, his migration was for that for which he migrated."
So if for example you "insulted" the Prophet (pbuh) by mispronouncing an Arabic word, you didn't really sin because it was not your intention to insult him in the first place. This was simply a mistake.